Usually we think of cells as differentiating only after they have ceased dividing. In Drosophila, however, germ cells characterized by conventional division differentiate into cystocytes that engage in a specific number of mitoses, each followed by incomplete cleavage. The result is a cluster of 16 cells joined by canals. This behavior is not restricted to insects, since cells that resemble cystocytes are found both during spermatogenesis and oogenesis in various mammalian species, including man. In the Drosophila system we have demonstrated that a temperature sensitive female sterile mutation exists that affects this incomplete cleavage, and we know that the transformation of gonocytes to cystocytes is triggered by a ring gland hormone. In the proposed research we plan to study the ultrastructure of cytokinesis while manipulating both the hormonal environment and the genotype of the germ cells. An in vitro system will be developed to which defined quantities of ecdysone can be added. We believe that the rims of the canals are derived from the contractile ring and will study the binding of H3- cytochalasine B to this organelle. These studies will elucidate some of the factors that regulate the organization of the plasma membrane relating to its ability to form a cleavage furrow following the mitotic division of the nucleus.